Compression Molding (Composites)

Compression molding forms fiber-reinforced composites in heated matched dies, ideal for medium-high volume, repeatable, moderately complex parts with good surface finish and strength.

Overview

Compression molding for composites uses heated matched metal tools and pre-weighed charges (SMC/BMC/prepreg) pressed into the cavity to form near-net-shape parts. The process delivers consistent fiber wet-out, good dimensional repeatability, and Class A surfaces when the tooling and material are dialed in. Cycle times are minutes rather than hours, so it suits medium to high production volumes.

Use composite compression molding when you need structural or semi-structural parts with moderate complexity, controlled thickness, and integrated features like ribs, bosses, and mounting points. It excels at large, shallow, plate-like geometries and shells: body panels, covers, housings, and brackets. Tradeoffs: high tooling cost, limited deep undercuts, and less freedom to locally tailor fiber orientation than hand layup or automated placement. Tolerances are good but not machining-tight, and post-machining is common on critical interfaces. For the right geometry and volumes, it offers an efficient balance of cost, mechanical performance, and finish quality.

Common Materials

  • Glass fiber SMC
  • Carbon fiber SMC
  • Glass fiber BMC
  • Carbon fiber prepreg charges
  • Glass mat thermoplastic (GMT)

Tolerances

±0.005" to ±0.010" on critical features, looser on large overall dimensions

Applications

  • Automotive body panels and exterior trim
  • Underhood covers and engine bay shields
  • Electrical and electronic equipment housings
  • Appliance and industrial equipment panels
  • Structural brackets and seat structures
  • Aerospace interior panels and fairings

When to Choose Compression Molding (Composites)

Choose composite compression molding for medium to high production of repeatable parts with moderate complexity, consistent thickness, and integrated ribs or bosses. It fits large panels, shells, and housings where you need good surface finish, decent tolerances, and structural performance without the cost or time of highly labor-intensive layup processes.

vs Resin Transfer Molding

Pick compression molding over RTM when you have higher volumes, can justify hard matched tooling, and want shorter cycle times with more automated charging and pressing. Compression molding is better when your geometry suits a two-sided die with moderate draw and you can work with sheet or bulk molding compounds instead of liquid resin systems.

vs Vacuum-Assisted Resin Transfer (VARTM)

Choose compression molding instead of VARTM when you need higher throughput, more consistent thickness, and better dimensional repeatability from hard matched tools. It is preferable for production programs where labor cost and part-to-part variability from flexible bagging and manual layup become major issues.

vs Prepreg Layup with Autoclave

Select compression molding over autoclave prepreg layup when you can trade some fiber-orientation freedom and ultimate performance for much lower cycle time and labor content. Compression molding wins for cost-sensitive structural parts and cosmetic panels where you can live with slightly lower specific properties but want faster, more automated production.

vs Prepreg Out-of-Autoclave (OOA)

Use compression molding instead of OOA prepreg when you want shorter cure cycles, more automation, and better thickness control from matched tools. It is advantageous when the part geometry is compatible with press tooling and program volumes justify the tooling investment versus more flexible but labor-heavy OOA layup.

vs Pultrusion

Choose compression molding over pultrusion when you need closed, non-constant cross-section shapes such as shells, brackets, or panels with varying geometry, ribs, and bosses. Compression molding handles 2D surfaces and integrated features far better, while still delivering good structural performance at scale.

Design Considerations

  • Maintain uniform wall thickness where possible; use ribs and gussets instead of thick sections to avoid sink, voids, and long cure times
  • Include adequate draft (typically 1–3° per side) on walls and cores to allow reliable ejection and longer tool life
  • Avoid deep undercuts and side actions; if unavoidable, clearly define them so the shop can design workable tooling and quote accurately
  • Use generous radii at corners and intersections to improve flow, reduce stress concentrations, and minimize fiber damage
  • Define critical machined surfaces and hole locations explicitly so the shop can plan secondary machining and realistic tolerance stacks
  • Specify gating, charge placement zones, and any required glass or carbon content/fiber orientation only to the level that truly matters to performance to keep cost down